1. Field of the Invention
The invention relates to methodology and apparatus for on-site production of nitric oxide (NO) and nitrogen dioxide (NO2), which yield water soluble nitrate ion (NO3−), and for bringing nitrate ions into contact with a water processing system, particularly in the context of an oil-field application.
2. Background of the Invention
Primary oil recovery generally yields less than 50% of what a given geological structure or reservoir contains. Accordingly, water injection is employed to enhance oil recovery from the porous rock formations that comprise many subterranean oil reservoirs. In enhanced oil recovery, a water processing system is used to inject a water solution into an oil reservoir. A water processing system can include above-ground facilities equipped with an apparatus or some facility that collects or distributes aqueous solutions, such as oil and gas wells, oil-water separators, water storage tanks, water treatment tanks, pipelines, and injection wells, water treatment facilities, or water transportation equipment. The injection process is known to produce hydrogen sulfide (H2S), however, which sours oil and gas reservoirs as well as the water processing systems and equipment associated with oil and gas recovery operations.
Hydrogen sulfide is produced by sulfate-reducing bacteria (SRB), which convert soluble sulfate (SO4) in the water processing system and oil and gas reservoirs to hydrogen sulfide. Such bacteria can arise during the drilling for oil, but they also may be present indigenously, before the drilling, and are known to be present in the aqueous phase of virtually all oil field operations. These bacteria and their affect on oil fields are described, for example, by J. R. Postgate, THE SULPHATE-REDUCING BACTERIA 2nd ed. (Cambridge University Press, 1984).
The contamination of oil and gas reservoirs and water processing systems by sulfate-reducing-bacteria (SRB) with hydrogen sulfide (H2S) has become a major operational problem and expense for the petroleum industry. The presence of undesirable amounts of hydrogen sulfide causes serious health and safety risks, severe corrosion of the equipment used to recover oil, and can drastically damage the production capabilities of the oil field by the formation of iron sulfide particles which precipitate and cause the clogging of recovery equipment and oil and gas reservoirs, thereby reducing oil volume and lowering the commercial value of the recovered crude oil. Accordingly, there has been intensive investigation directed at preventing the formation of hydrogen sulfide and/or removing the hydrogen sulfide once it is produced in oil field applications.
Treating the affected oil and gas reservoirs and water processing systems with nitrate is effective in degrading existing H2S and preventing further occurrence of H2S. For example, it is known that the addition of nitrate and nitrate compounds to a system containing SRB will reduce the amount of SRB in the system and thus the amount of hydrogen sulfide formed by SRB. This method relies on strains of Thiobacillus denitrificans and other denitrifying microorganisms that are present in oil field waters. For example, hydrogen sulfide present in a water processing system is removed and the production of hydrogen sulfide by sulfate-reducing bacteria is prevented by introducing nitrate into the system, whereby denitrifying microorganisms utilize the nitrate and produce several mechanisms and conditions that prevent the SRB from producing hydrogen sulfide.
Nitrate for this purpose is typically manufactured by oxidizing ammonia or mined by conventional practice, and the resultant dry nitrate is transported, blended into liquid solutions, and stored in close proximity to an oil field or other remote site for use. The logistical chain of manufacturing and movement of dry nitrate compounds through the supply chain has many safety and health issues and has proved to be needlessly expensive while leading to shortages and intermittent supply of nitrate to the point-of-use, or no supply at all. Currently, nitrate is (1) sourced in dry form from manufacturing plants, (2) transported by various means including rail, trucking, and oceanic shipping and (3) stored in warehousing until needed.
Transporting and storing large quantities of dry nitrate, or blends of nitrate in solution raises numerous safety and cost issues. Once needed, nitrate is transported to blending plants to blend with water as a useable product and transported again to supply oil and gas production operations that are most often located off-shore and in remote on-shore locations far removed from the origin of nitrate manufacturing. Further, large quantities of nitrate must be stored on-site.
The conventional mode of nitrate manufacturing uses the Haber process and is dependent on a constant source of natural gas as the critical component of manufacture. Manufacturing plants are dependent on the price for natural gas which has a direct relationship to the cost of nitrates. New Haber-type manufacturing plants require several hundreds of millions of dollars and several years to construct and must be strategically located near reliable sources of natural gas. Transportation, storage, and blending costs have also increased significantly. Consequently, nitrate prices are at historical highs and are expected to keep increasing with time as the demand continues to grow.
Accordingly, the need exists for an economical and effective means to produce nitrate locally and to bring it into contact with a water processing system, in order to prevent the formation of hydrogen sulfide and/or to remove any existing hydrogen sulfide in the water processing system or an oil and gas reservoir supplied by the water processing system. Further, there is a need for means to produce nitrate locally and to bring nitrate into contact with a water processing system that is useful in the recovery of oil; this, so that H2S contamination will not adversely affect the reservoir or equipment used in the oil recovery process.